aio info

Running nginx on freebsd 8.2 with 16 workers and aio on, when the server encounters heavy writes, all processes are in D state
Aio queue is way below system/process limit
A request for small file (said file will be in fs buffer/cache) every second varies wildly from 1ms to over 1s
>From man ps:
D Marks a process in disk (or other short term, uninter-ruptible) wait.
What's causing the lock?
Many thanks
Richard